The invention relates to an electromagnetically shielded cabin.
To shield highly sensitive electronics means, computer systems, etc. against high-frequency electromagnetic waves or field, shield cabins are used. The cabins offer both a shielding against the irradiation or radiation of electromagnetic radio or radar waves and against electromagnetic interferences e.g. by strokes of lightning. The cabins also protect the electronic means against a destruction by a nuclear electromagnetic pulse, i.e. against the strong electromagnetic field which occurs during a nuclear explosion, in particular if they are used in the military field.
A weak point of the shielding is the abutting area between a stationary component, i.e. a stationary wall and a movable wall component such as a door, a flap, a hatch or a movable window. However these abutting areas must also achieve the necessary shield attenuation values with the wall being closed and the transfer resistances may only be in the milliohm range. Therefore it is necessary to establish a good electric contact across the abutting area.
An electromagnetically shielded cabin of the type explained above is known from DE-A 26 01 277. The cabin has a sealing system between a door frame of thick sheet metal, e.g. plated steel and a door of a steel construction. The sealing system consists of a knife contact of plated steel which is inserted between contact springs made of beryllium copper. The sealing system of the known cabin does however only achieve sufficient attenuation values with the described combination of materials according to the statements of this publication, and it is explicitly mentioned that these attenuation values can by no means be achieved when using aluminum.
In the publication of Siemens company "EMV Raumabschirmung, Kabinenteile, Filter", edition 83/84, a sealing system which can be used for the abutting area is described which is called a knife contact. Part of this sealing system consists of two contact springs disposed in a groove, the groove having a substantially rectangular cross-section. The second part is a beaded, strip-shaped contact web with an oblong, rectangular cross-section which angles off from the door frame. If the door is closed, the beaded area of the contact webs is pressed in such fashion between the contact springs that the same are slightly deformed. Due to this, each abuts against the walls of the groove and the surface of the contact web in a conductive contact. The cabin consists of a frame of sections and a sheet steel filling. The contact web consists also of steel. In order to ensure that the electric contact is not deteriorated in the course of time across the abutting area, the sealing system must be cleaned at regular intervals and protected against corrosion. This rust protection is also achieved by a thin fat film, which however, will increase in turn also the transfer resistance.
The problem of the cleaning and the upkeep of the sealing system becomes especially serious, if mobile cabins are to be built of aluminum to save weight. The aluminum forms immediately an oxide film on its surface upon contact with the air oxygen which increases the transfer resistance. To achieve acceptable shielding values, such a sealing system of aluminum would have to be cleaned daily, which is neither reasonable nor is it carried out in practical operation.
The invention is thus directed toward making an electromagnetically shielded cabin with a sealing system available, which requires little maintenance even if aluminum is used as construction material for the cabin and achieves high shield attenuation values.
The object is accomplished by providing a sealing system for an abutting area between a stationary component of the cabin and a moving wall component wherein two spaced, bent contrast springs are connected to the one component, between which a contact web connected to the other component can be passed, and contact surfaces in each case being formed between the contact web and the contact springs and between the contract springs. The electromagnetically sealed cabin of the present invention is characterized in that at least one of the contact surfaces is comprised of aluminum which is provided with a tin coating.
According to the present invention, using a tin coating achieves very good contacting between the contact surfaces being in engagement with each other. The obtainable shield atentuation values are noticeably above the relevant standard even if aluminum is used as construction material for the wall and/or the door. The shield attenuation is also not deteriorated during continuous operation, which is surely attributable to the fact that tin is a relatively soft metal so that oxide films are removed due to the relative movement between the contact surfaces, the contact surfaces are thus kept uncovered and a form-fit contact seat is properly worked out. At the same time tin only forms an extremely thin oxide layer even under a great environmental strain which does not hinder a good contacting.
So far tin has only been used for purely electrical connections, for instance for permanent connections or as a coating in clamping connections which must only rarely be loosened. The US-A-2 858 520 describes a terminal of an aluminum material which is provided with a cadmium layer above adhesion promoting layers of zinc and copper. In order to reduce the cyclically occurring heating by the cadmium layer which occurs in the case of a-c load, a thin porous film of preferably nickel, but also of tin was applied externally on the cadium layer. The cadmium layer is to be in electrical contact with the clamped wire through the pores of the porous film. However, this requires that the terminal be tightened with such a great contact pressure that the contact points interflow. The contact terminal is thus only destined for a single use. Indications that a tin coating can also be used for sealing systems of electromagnetically shielded cabins are not revealed by this publication.
The DE-A-33 05 918 shows furthermore a connector for printed circuits which is designed in the form of a knife contact. One of the two contact surfaces of the knife contact and the abutting contact surface of the contact spring are in each case coated with a noble metal such as gold or palladium, while the opposite contact surface of the knife contact and the abutting contact surface of the contact spring are coated with tin. A spring material is indicated as base material for the spring and brass is indicated for the knife contact. Coatings of noble metal are, however, much too expensive in the case of the large sealing surfaces for the sealing system of an electromagnetically shielded cabin. The tin coating and the coating with noble material are moreover connected with each other in electrically conductive fashion which may lead to contact corrosion in particular when used in the out of doors (high air humidity and rain), in which the base metal is eroded. Moreover a tin coating on both sides is considered to be detrimental in the publication.
It is a further object of the invention to improve the adhesion of the tin coating on aluminum surfaces by arranging a coupling agent between the tin coating and the aluminum surface. If zinc is used as a coupling agent, a comparatively good abrasion resistance will result.
It became apparent that the contact is still further improved if according to the contact springs are also tin-plated, even if they consist of a material other than aluminum.
To apply the tin coating the flame-spraying process is preferably used according to claim 5.
A wedge-shaped cross-section of the contact web is preferred for allowing the contact surfaces between the contact web and the contact spring to rub against each other across a greater area. Moreover, the flattening of the curvature of the bent contact spring is supported by this cross-section so that the same moves rubbingly along the contact surface of the element supporting the contact spring with its contact surface. Due to this frictional movement a better self-cleaning is achieved both between the contact web and the contact spring and between the contact spring and the element supporting it. The wedge-shaped cross-section is further advantageous in that the pressing pressure against the contact springs can be varied thereby and work tolerances can be compensated thereby. However, due to the wedge-shaped cross-section, the closing of the door, in particular in the area of the hinges, is facilitated, where the movement path of the contact web or the gap between the contact springs is strongly bent due to the greater vicinity to the fulcrum. Due to the inclination of the surfaces of the contact web, the curvatures can be compensated substantially better.
In order to protect the contact springs against damage and to facilitate a correct closing of the door, the contact springs are disposed in the groove.
An aperture angle of the wedge cross-section, in the range of 25.degree. to 35.degree. will largely avoid the self-inhibition of the contact web.
It is a further object of the invention to provide contact springs adapted to be simply clipped in so that they can be easily disassembled and assembled.
An example of embodiment of the invention is explained in more detail in the following by means of the drawings.